Spectrophotometry

ABSTRACT

A robust spectrophotometer (also known as a color spectrometer or colorimeter) is self contained in a housing which is adapted to be held-held and has all of the electrical, optical and electro optic elements mounted on a board captured within the housing at one end of which light from a sample is restricted to an object area and projected after being dispersed spectrally, as with a reflection grating, to an image area at a photodetector via a lens which has an optical axis and converges the dispersed light at the image area. The dispersive element is mounted on an arm having a pivot laterally offset from the dispersive element&#39;s surface where a diverging beam of light from the object area is incident and is deflected to the image area. The geometry is such that the dispersive element may be rotated to a position where the beam is specularly deflected (zeroth order diffraction), and the spectrometer calibrated when the dispersive element is in the specular reflection/deflection position. The path from the object area is approximately perpendicular to the optical axis, and then is folded by mirrors to direct the beam to incidence on the dispersive element, from which the beam is deflected and focused by the lens, the focal length of which is such that the image and object areas are in conjugate relationship. A pivotal foot on the housing having an aperture may be used to facilitate alignment of the sample with the entrance opening to the housing of the spectrophotometer.

This is a continuation of application Ser. No. 08/471,617 filed May 22,1995, now U.S. Pat. No. 5,684,562, issued Nov. 4, 1995, which is acontinuation of application Ser. No. 08/210,806, filed Mar. 18, 1994,now abandoned.

DESCRIPTION

The present invention relates to spectrophotometry and more particularlyto apparatus for measuring the spectrum of optical illumination at aplurality of successive wavelength increments.

The invention is especially suitable in use for color management toenable colors to be matched for consistency with accepted colorstandards and for balance so that images formed by color reprographics(especially by digital or computer controlled imaging techniques) canmeet applicable standards. Spectrophotometry apparatus and methods whichare practiced in accordance with the invention may also be useful in thecalibration of radiant sources such as the screens of color monitors(phosphors of which are electronically activated to produce color imagesin such monitors.) The spectrometer apparatus may also be used as adensitometer and generally as a color spectrometer for color matchingpurposes as for example in mixing of paints. Such color matching anddensity control have been called color management in the reprographicsfield.

The wide and general use of spectrophotometers in color management hasbeen limited by their cost, and the size and weight needed forarrangements of optical elements to provide the requisite accuracy ofspectral measurements. Reducing the electrical and optical elements to asize which can be held-held and easily portable by an operator, withoutsacrificing accuracy of spectral measurements has been a goal, which nowis achievable because of the invention. In addition to cost and size,another problem is that such held-held spectrophotometry instruments arelikely to be dropped onto hard surfaces, and thus need to be robust,again without sacrificing accuracy.

Accordingly, it is the principal object of the present invention to makeimprovements in spectrophotometry, which enable spectral measurements ofoptical sources, whether radiant (optically emissive) or reflective, andwhich provide information as to the spectral characteristics thereof, tobe made with sufficient accuracy for color management purposes by meansof an instrument which is adapted to be held-held and portable.

It is a further object of the present invention to provide improvedspectrophotometry apparatus which is both self-contained and portable,and which provides accurate spectral measurements.

It is a still further object of the present invention to provide aninstrument which may be fabricated at a cost lower than instruments forthe purpose which are presently commercially available and, althoughself-contained and of a size adapted to be held-held, which providesmeasurements of sufficient accuracy to be generally useful in colorreprographics, calibration of computer monitors and other luminescentsources, and generally for color management purposes.

It is a still further object of the present invention to provideimproved spectrophotometry apparatus which is adapted to be used eitherin a dispersive or specular reflective mode so as to obtain measurementsof the intensity of luminescent sources as well as the spectrum thereof,thereby enabling such sources, such as color monitors to be calibratedso as to provide images with color accuracy and balance.

Briefly described, the invention improves the art of spectrophotometryby providing an instrument useful for measuring the spectrum of lightwhich the instrument receives through the use of a dispersive elementrotatable in angular increments, particularly increments correspondingto angular displacements of the shaft of a drive motor which effectsmovement of the dispersive element about an axis of rotation laterallydisplaced from the element. Optics provides a geometry which illuminatesthe dispersive element with light from a sample which enters thespectrophotometer at an object area. These optics include an aperturewhich defines the object area and which is desirably in the form of arectangular slit. The aperture forms the illumination into a beam whichdiverges as it propagates along a first path between the object area andthe dispersive element, on a surface of which the beam is incident. Thebeam is then deflected (diffracted where the dispersive element is agrating, as is preferably the case). The deflected beam propagates alonga second path to a photodetector at an image area which terminates thesecond path. A lens having an optical axis is disposed along the secondpath. The dispersion in the dispersive element spatially distributes theillumination in accordance with its wavelength so that the chief ray ofthe illumination, which has a wavelength in the middle of each opticalincrement, extends along the optical axis regardless of the angularorientation of the dispersive element as it scans the wavelength of theillumination. The rays of wavelength at each end of the wavelengthincrements are at opposite sides of the beam and are focused at oppositeends of the image area by the lens. Thus, the entire spectral incrementis measured and translated into a corresponding electrical signal by thephotodetector.

In order to control the dynamic range of the signal produced by thephotodetector, it may be desirable to provide an optical attenuatingelement which moves with the dispersive element across the first path,intercepting and attenuating the beam as a function of the angulardisplacement corresponding to the wavelength increment which is beingmeasured. Then signals at one end of the spectrum (the longer wavelengthend) are reduced in amplitude, relative to the signals obtained from theshort wavelength end, without the need for electronic means forcontrolling the dynamic range, thereby providing outputs from thephotodetector which correspond to the color perception of light. Thelight entering the object area is diffuse and may be imaged at a Fouriertransfer plane (focal plane) of a field lens at the object area. Then, ashutter which moves with the dispersive element may be used as theattenuating element.

The dispersive element is desirably a plane diffraction grating which isoperative in the -1 diffraction order to deflect the light in eachwavelength increment so that the rays at the center wavelengths (chiefrays) are generally along the optical axis of the lens. The opticspermits the grating to be rotated to a position where the zeroth orderdiffraction or specular reflection from the grating occurs andilluminates the photodetector. In this position, the spectrophotometermay be used to monitor the intensity of the illumination and to obtainthe intensity characteristic of a luminescent source, for example, acomputer monitor; the phosphors of which emit at different intensitiesdepending upon the intensity of the electron beam which excites thephosphor in the monitor. By varying the intensity of the electron beam(the current of each beam) in steps, the intensity characteristic of themonitor, and particularly of each color of the phosphor, may be measuredso that the monitor may be adjusted to provide accurate color balanceacross its entire image intensity range.

The foregoing and other objects, features and advantages of theinvention will become more apparent from a reading of the followingdescription in connection with the accompanying drawings in which:

FIG. 1 is a plan view showing the arrangement of the optical andelectro-optical components of spectrophotometer apparatus which isprovided in accordance with the invention;

FIG. 2 is a perspective view illustrating a held-held and self-containedspectrophotometer which utilizes the spectrophotometry apparatusillustrated in FIG. 1;

FIG. 3 is a side view of the spectrophotometer shown in FIG. 2;

FIG. 4 is a front view of the spectrophotometer shown in FIGS. 2 and 3,the view being taken from the right in FIG. 3;

FIG. 5 is a sectional view which is taken along the line 5--15 in FIG.4, and shows the spectrophotometer apparatus of FIG. 1 containedtherein, also as viewed along the line 5--5 in FIG. 1;

FIG. 6 is a sectional view taken along the line 6--6 in FIG. 4 and alsoshowing the spectrophotometer apparatus of FIG. 1 taken along the line6--6 in FIG. 1;

FIG. 7 is a schematic diagram, in the form of a layout, of thecomponents of the spectrophotometer apparatus shown in FIG. 1 insimplified form;

FIG. 8 is a view similar to FIG. 7 but with the dispersive element ofthe spectrophotometer angularly displaced to a position for specularreflection (zeroth order diffraction) of the light incident thereon;

FIG. 9 is a view similar to FIG. 1, but showing the dispersive elementand the arm mounting that element and a shutter carried on the arm intwo different angular positions where two different wavelengthincrements of the spectrum of the illumination entering thespectrophotometer, which correspond to these two angular increments orsteps, are measured;

FIG. 10 is a schematic diagram of an embodiment of the photodetector forselecting samples of smaller and larger size for spectral analysis.

Referring first to FIG. 1 there is shown a top view of optical,electronic and electrical components of a spectrophotometer embodyingthe invention which components are mounted on a plate provided by acircuit board 10 of insulating material having on the bottom sidethereof, electronic components, principally integrated circuit chips,which are connected by printed wiring to the electrical and electroniccomponents on the board 10 and to a connector which connects, via acable, the spectrophotometer to equipment with which it is used, such asa computer. The cable is sufficiently long and flexible to enable thespectrophotometer to be hand-held and portable. The connector is notshown in the drawing to simplify the illustration. The componentsinclude a stepper motor 12 which is mounted on the bottom of the boardand has a shaft 14 projecting through the board. The shaft is infrictional driving contact with an elastomeric, resilient layer 16 at anend of an arm 18 which is rotatable about an axis of a pivot 20 on thetop of the board 10. The profile of the arm 18 at the end having thelayer is circular, but may be made non-circular (cam shaped) to matchthe wavelength increments more closely to equal angular increments orsteps, through which the shaft 14 turns when making a spectral responsemeasurement.

Mounted on a bracket 22 at the end of the arm on the opposite side ofthe pivot where the resilient elastomeric layer 16 is attached, is adispersive element, which is preferably a diffraction grating 24, whichhas grating lines on a face 26 thereof. These grating lines may beprovided by blazing and are designed in accordance with conventionalgrating design techniques to deflect with maximum energy in the -1diffraction order. Other dispersive elements such as prisms and otherdiffractive orders may be used.

It will be understood that the spectrophotometer may be used with thegrating operative in the zeroth order where light incident on the face26 is specularly deflected. While a reflection mode diffraction gratingis preferred in this embodiment of the invention, another type ofdispersive element may be used. The optics may also be usable in thetransmission mode of the grating. However, the use of the transmissionmode prevents the folding of the optical path and is not presentlypreferred, in the interest of providing a spectrophotometer of miniaturesize and weight. In this connection the herein illustratedspectrophotometer may be less than about 5 inches long and 5 inches wideand less than about two inches thick and have a weight of less than 1pound thereby enabling it to be hand-held and portable.

The spectrophotometer is responsive to light from a sample which isspaced in the vicinity of an entrance end thereof at a locationindicated at 28 in FIG. 1. This sample area may be a patch or testregion on a photograph or other image to be reproduced. The spectrometermay also be used to measure the color content (spectral characteristics)of radiating optical sources, such as electro-luminescent devices. Suchsources may be the screens of computer monitors, which are cathode raytubes having phosphors which are excited by electron beams; thephosphors being on the screens of the cathode ray tube of the monitor.

In the event reflective patches or materials are to be analyzed forcolor or spectral content, they may be illuminated by lamps 30 and 32.Small lens end, incandescent lamp bulbs are preferably used as the lamps30 and 32 to provide the illumination of a reflective sample.

An object area of the spectrophotometer is defined by a aperture orobject slit 34, which is preferably a rectangular slit aperture with theshort dimension of the slit LD defining the width of the object area.The long axis or length of the rectangle of the aperture isperpendicular to the plane of FIG. 1. The object area and the apertureare co-extensive. The light which reaches the aperture arrives from theentire sample area in a region defined by LD along the one dimensionthereof. This light is diffuse and enters the object area (the aperture34) from multiple directions.

The width of the object slit 34 and the image aperture (anotherrectangular slit) 64 which defines an image area determine thewavelength resolution. The width of the image slit is determined by theangular dispersive strength of the dispersive element and the focallength of the imaging lens. For a diffraction grating, the angulardispersive strength is obtained by differentiating the well knowndiffraction formula: sinθ₁ +(λm)/p=sinθ₂, where θ₁ is the incident angleonto the grating, λ the wavelength of illumination, m the order ofdiffraction, p the grating pitch, and θ₂ the diffracting or diffractionangle. Differentiating this equation with respect to λ and θ₂, theangular dispersion of a grating is obtained: Δθ₂ =Δλm/(p cosθ₂). In thepreferred embodiment, the diffracted angle is such that cosθ₂ ≧0.99 overfrom λ=390-700 nm. Consequently the angular dispersion is approximatelyconstant over the visible spectrum.

In the preferred embodiment where the grating is juxtaposed between theobject and an imaging lens 66, the image slit 64 width is the angulardispersion of the grating multiplied by the distance from the secondprincipal plane of the lens to the image of the object slit 34. Forexample, using a 30 mm focal length lens and a 1211 lp/mm (line per mm)grating with an object distance of 159.69 mm, the image distance is36.94 mm, and the image slit width is 0.44 mm for 10 nm resolution imageside resolution.

The total resolution of a spectrometer is the convolution of object-sideand image-side wavelength resolutions. The optical throughput andresolution is optimized when the object-side and image-side wavelengthresolutions are matched. This is achieved when the width of the image ofthe object slit equals the physical size of the image slit 64. Thismeans that the angular width (perpendicular to the diffraction grooves)of the object slit, and image slit when viewed from the principal planesof the lens, are equal.

The magnification of the object slit to the image slit is anamorphic forprismatic and diffractive dispersive elements. Perpendicular to thedirection of dispersion, the magnification is given by standard imaging:magnification=ratio of image to object distance. Parallel to thedirection of dispersion, the magnification is given by the ratio ofimage to object distance multiplied by the angular magnification inducedby the dispersive element. For a grating, the angular magnification isthe ratio cosθ₁ /cosθ₂. Therefore, an object slit 2.69 mm×9 mm is imagedto 0.44×2.08 mm using a 30 mm focal length lens and a 1200 lp/mmgrating.

The height of the object slit is determined by four factors, the size ofthe emissive or illuminated sample area, the size of the opticalelements (their apertures), the aberration tolerance of the system andthe detector size. In the presently preferred embodiment, the aperturestop size is 12 mm and the object height is 9 mm.

In accordance with a feature of the invention the dynamic range of thespectral measurements may be controlled using a shutter 36 which ismounted on the arm 18. Then, it is desirable to provide a field lens 38on the inside of the aperture 34 across the object area. This field lensmay be a plano-convex lens having a focus at the location of the shutter36. In other words, the shutter moves across the Fourier transfer planeof the lens 38. The lens 38 converts the angular distribution of lightat the objects slit 34 into spatial distribution of intensity at thelocation where the shutter 36 interrupts the beam 44. Accordingly theshutter effects the spatial distribution of light from the sample area28 uniformly. In the event that it is not desirable to use a field lensand shutter for attenuating the light to control the dynamic range ofthe spectrophotometer, a variable neutral density filter may be used asthe attenuating element.

A bracket 40 supports the aperture 34 and another aperture 42 whichserves as a baffle to restrict the size of a beam 44 of illumination,which diverges as the beam propagates away from the aperture 34 along afirst path which terminates at the surface 26 of the grating 24. Thelength of this path is such that the grating is substantially filled onthe face 26 thereof, at least along the LD dimension of the beam. Inorder to ensure such filling and also to coordinate the angular steps ofrotation of the motor shaft 14 with the spectral increments beingmeasured, the beam path length between the object slit 34 and the face26 is effectively lengthened by means of optics including foldingmirrors 46 and 48. These mirrors 46 and 48 are mounted on the topsurface of the board 10 in a bracket 50 so that the beam path remainsfixed, but sufficiently long to fill the face 26 of the grating. Thechief (approximately central) ray 52 of the beam along the first pathand the rays 54 and 56 spatially displaced at opposite ends of thedimension LD are indicated in FIG. 1.

The beam is deflected by the grating 24 in accordance with thewavelength of the illumination by different amounts in accordance withthe period of the grating (the distance between the grating lines) andthe wavelength of the illumination. The equation describing thedispersion of a grating is sinθ₁ +(λm)/p=sinθ₂, where θ₁ is the angle ofincidence. θ₂ is the angle of diffraction, λ is the wavelength of light,m is the order of diffraction, and p is grating period. The period andthe angle of incidence of the beam on the surface 24, for example asmeasured by the angle between a perpendicular to the surface 26 and thechief ray 52, is selected so that the chief ray 58 of the deflected beam60 has a wavelength equal to the wavelength at the center of eachspectral increment or step (corresponding to each angular step ofrotation of the motor shaft 14) and this chief ray 58 is always, foreach spectral increment regardless of the tilt of the grating 14, alonga line which is imaged to a photodetector 62. This line is along asecond optical path which is between the face 26 and the photodetector62. The photodetector defines an image area or may be used with anaperture 64, rectangular image slit discussed above, to define the imagearea of the spectrophotometer.

The lens 66, preferably an achromat lens 66, is mounted on the top ofthe board 10 and converges the deflected beam so that it substantiallyfills the aperture 64, with light from the spectral increment. The rays68 and 70 along opposite sides of the deflected beam are of wavelengthsat opposite ends of the wavelength increment. These rays are convergedby the lens 66 which has one focus at the image area and the other, orconjugate, focus at the object slit 34.

Because each increment is small, lateral chromatic aberration may bedisregarded in the spectrophotometer design. Longitudinal chromaticaberration may be also be compensated since the path length between theface 26 of the grating and the lens 66 varies as the grating tilts. Byselecting the length of the first path between the object slit 34 andthe grating 24, longitudinal chromatic aberration may also becompensated. The use of an achromat 66 is preferred, since the achromatis designed to compensate for longitudinal aberration which may bepresent even with the automatic path length compensation due to thegeometry of the beam along the first path from the object slit 34 to thegrating 24.

The grating is tilted according to the grating equation sinθ₁+(λm)/p=sinθ₂ in order to scan the spectrum across the image slit 64.For all grating orientations, the incident chief ray and the diffractedchief ray are separated by 45°. For p=0.833 μm (1200 lp/mm), the angleof incidence θ₁ is 49.6°, for λ=700 nm to be centered on slit 64, andthe angle of diffraction θ₂ is -4.55°. The angle of incidence θ₁ is37.2° for λ=390 nm to be centered on slit 64, and angle of diffractionθ₂ is 7.8°. Tilting the grating in 0.399° steps scans the spectrumacross the slit 64 in approximately 10 nm steps. Scanning the rangeλ=700-390 nm requires a total of 32 steps. Due to the non linearity ofthe grating equations, there is a small variation in the size of thewavelength steps when scanning the grating in equal angle steps. The RMSerror in the wavelength step is 0.24 mm over the range of 700 nm-390 nm.This residual error can be corrected by varying the radius of thetraction surface 16 to form a cam and spring loading the motor such thatthe motor shaft follows the cam surface.

It is desirable to start each measurement at a index or referenceposition with the arm tilted to one side or the other, preferably thelower wavelength end of the spectrum. It will be appreciated that thelocation of the grating and the arm 18 is shown for an increment, ofboth the angular step and the spectral increment or bin, in the middleof the spectrum. Then a notch 71 in the arm 18 serves as an indexingdevice to operate either a switch or a photo interrupter which providesa signal to the motor control circuitry when the motor is indexed to itsstart or home position.

Each wavelength band or increment is imaged at the image area and thephotodetector provides an output level corresponding thereto. Thisoutput level may be digitized into a multi bit digital byte or number,for example, of 16 bits, which defines the spectral intensity with aresolution of approximately 65,000 levels. This digital number may becreated by utilizing the output of the photodetector to control thefrequency of an oscillator and to measure the number of counts or cyclesor half-cycles of the oscillator's output (repetition rate) in a counterwhich provides the digitized spectral output for that increment or step.

It may be desirable such as shown in FIG. 10 to utilize threephotodetectors 62A, B and C. All photodetector's outputs are used when acontrol signal sets a group of amplifiers in a sum circuit 74. Then theeffective area under observation is maximized. When the control signalonly selects the center photodetector 62B, the sample area is minimized.Note that this configuration changes the height of the sample area anddoes not effect the spectral resolution. The output of the summingcircuit is amplified in amplifiers 76 and 78. The amplifier 78 has 3times the gain of the amplifier 76 to compensate for the use of aphotodetector area of 1/3 that of all three areas 62A, B and C together.The outputs from the amplifier are then applied to a signal processorand digitizer to obtain the digital signals corresponding to thespectral. This signal processor processes the analog signals and thendigitizes them and provides the output to a computer or other dataprocessor 82.

The illumination at the red or high wavelength end of the band isdetected with greater efficiency by silicon photodetectors than thesignals at the low or blue end of the band. In addition incandescentlamps have greater spectral intensity in the red than the blue. A widedynamic range, for example, over 100 times larger at the red end than atthe blue end may need to be accommodated in order to cover the actualintensities involved. In order to reduce this dynamic range, theattenuating element 36 comes into play. Then, at increments in the redend of the band, more attenuation is inserted than at the blue end ofthe band where the attenuation is minimal. This automaticallycompensates for the differences in intensity at opposite ends of theband and reduces the dynamic range which must be accommodated by thecircuitry operative with the photodetector 62.

Another method of normalizing the detected spectral intensity is use aanalog to digital converter to sample and digitize the detectedelectrical signal from the photodetector. The conversion speed of theanalog to digital converter is fast enough to digitize the electrical atleast 16 times before the motor increments to the next spectral bin.Signals from each spectral bin are sampled multiple times and summed.Wavelengths from 440-410 nm are digitized eight times and summed.Wavelengths from 410-390 nm digitized sixteen times and summed. Thenumber of times each wavelength bin is digitized is stored with thewhite and black calibration data. Digitizing the lower intensity binsprovides two benefits. The first is that it increases the effectivesignal level of the low intensity bins. Second, the multipledigitizations average out the digitization noise of the analog todigital converter. At lower signal levels the digitization noise has alarger percentage effect on the detected signal than at higher levels.

FIG. 7 simplifies FIG. 1 by removing the attenuating element and showsthe motor shaft 14 in friction driving contact with the resilientmaterial covered end of the arm 18. This frictional driving relationshipis again illustrated at the center of the spectral range. The arm 18 hasa lower extension or heel 80 at its resiliently covered end whichextends the angle over which the arm 18 and the grating may be tilted.

The arm located at the largest tilt angle of the grating is shown inFIG. 8. This is the angle where the angles of incidence and deflection,both measurable between the center ray and displaced center ray andperpendiculars to the surface 26, are equal. At this position, thegrating is operative in its zeroth order mode as a reflector and thebeam is specularly reflected and projected through the lens 66 to thephotodetector 62. When in the position shown in FIG. 8 thespectrophotometer may be used to measure the intensity of the light atthe sample area 28. In this position, the lamps 30 and 32 may becalibrated by using a white body (totally reflective) at the sample area28. The current which changes the intensity of the lamps 30 and 32 maybe stepped through a range over which the lamps are safely operablewithout burning out or having excessively reduced life. Moreover, eachlamp may be separately monitored. The current which illuminates thelamps is then adjusted to provide the calibration, sometimes called thewhite point calibration. Similarly, a black sample area 28 may be usedand the black point intensity calibrated. When the spectrum is measuredby stepping the motor through its angular increments and measuring thephotodetector output at each step, the photodetector may process theoutput by subtracting the black point calibration value and ratioing(dividing) the outputs at each spectral increment or bin by the whitepoint calibration value, thereby normalizing the outputs.

The calibration at the specular or zeroth order reflection positionshown in FIG. 8 may be also be used to measure the variation inintensity with operating current to each gun which produces each beamand excites the different phosphors, (the red, blue and green phosphors)of a monitor. A calibration curve of the intensity versus gun currentmay be obtained and used together with the spectral characteristics asmeasured with this spectrometer over each of the angular increments inorder to calibrate the monitor and balance the colors produced by thephosphors.

FIG. 9 shows a view similar to FIG. 1 with the arm shown in full linesand in dash lines at opposite ends of its angular range, that is whenmeasuring the spectral increments or bins at opposite ends of thespectral range; for example, the bins with central band widths of 390and 700 nm. It will be noted that the first path or leg 84 of the beam44 has its central ray 52 perpendicular to the optical axis 58 of thelens 66 irrespective of the tilt of the grating 24. The Fourier transferplane of the field lens 38 is shown at 86 in FIG. 9.

The self contained hand-held spectrophotometer provided by the inventionis also shown within its housing 90 in FIGS. 2, 3, 4, 5 and 6. Thehousing is made of two shells, namely a bottom shell 92 and a top shell94, both of plastic, which interlock with each other and capture thespectrophotometer apparatus including the board 10 and the componentsthereon within the shells, and particularly on posts or stand-offs 96internally of the shells. These support, as by clamping, thespectrophotometer apparatus at spaced locations on the board 10, and atthe support bracket 50 for the mirrors thereby mounting the board andthe spectrophotometer optics in fixed position inside the shells. Thehousing 90 has opposite ends which are a front or object viewing end 130and a rear end 131.

The lower housing shell has a heel 98 with slots 100, and 102 whichreceive tongues, one of which 104 is shown in FIG. 5, all on the rearend 131. Between the slots 100 and 102 there is disposed an opening 105in which an indicator, for example a light emitting device (LED) may belocated which is illuminated when the apparatus is turned on. Thecircuits on the board 10 also connects to a connector 106 which providesa terminal for the cable (not shown); but discussed above. Only theopening 105 for the LED is shown in FIGS. 4 and 6 to simplify theillustration.

The top of the top shell has indentations 110 and 112 for the fingers ofthe operator a, while the operators thumb may be placed against aU-shaped section 114 of the bottom shell 92. A switch 116 may bedepressed by a flexible section of the U-shaped shoe 114 (a cowling) toenergize the spectrophotometer. When the lamps 30 and 32 are to beenergized, the switch may be depressed either twice or to a greaterextent than when applying power to the other spectrophotometercomponents.

The shells 90 an 92 are assembled by means of overlapping posts orstanchions 120, 122 on the top shell and 124 and 126 or the bottom shell92 all at the front on sample viewing end 130 of the housing 90. A rodor pin 128 extends through the overlapping post or stanchions 122, 126and 120, 124. At the end 130, there is provided an entrance opening 132for light from the sample. The sample is placed against the sole 134 ofa foot plate 136. Preferably, the foot plate is snapped over the rod 128and therefore is pivotally mounted on the housing so that a section 138thereof having an entrance opening aperture 140, may be pivoted to bearagainst the front end 130 of the housing as shown in FIG. 6. Severaldifferent foot plates having apertures 140 of different size may beused, including one carrying a fiber optic to an extender which may beplaced on a monitor, as with a suction cup.

In operation, the sole of the foot plate is placed against the sample.At this point in time, the foot plate may be tilted clockwise away fromthe first position shown in FIG. 6 to a second position so that itslower section bears against the inwardly sloping portion 138 of the topshell 94. The entrance opening 140 is then exposed for observation. Thenthe light from the lamps 30 and 32 may be visible, as a beam projectingthrough the entrance opening 132 in the housing 90. Thespectrophotometer unit can be located or aligned with the sample areawhich is to be analyzed. Before the measurement is initiated, the footplate 136 is brought to the position shown in FIG. 6. Then the switch116 may be depressed to operate the motor, index the grating 24 to itshome position, and begin the sequence of successive angular steps torecord the spectrum of the illumination from the sample area.

From the foregoing description it will be apparent that there has beenprovided improvements in the field of spectrophotometry (also known ascolorimetry and color spectrometry) and particularly and improvedspectrophotometer instrument. Variations of modifications in the methodsand apparatus described herein, within the scope of the invention, willundoubtedly suggest themselves to those skilled in the art. Accordinglythe foregoing description should be taken as illustrative and not in alimiting sense.

We claim:
 1. Spectrophotometer apparatus comprising means forpresenting, at an object area having first and second dimensionstransverse to each other, a wavelength increment between lower andhigher ends thereof, across said first dimension of said object area,photodetector means responsive to light from said object area acrossboth said first and said second dimensions, and means for controllingthe extent of the area in said second dimension over which saidphotodetector means is responsive.
 2. The apparatus according to claim 1where said first dimension is the width of said area and said seconddimension is the height of said area.
 3. The apparatus according toclaim 1 wherein said photodetector means comprises a plurality ofphotodetectors exposed to said area and respectively providing differentones of a plurality of separate outputs, one of said plurality ofphotodetectors being disposed at the center of said area and the othersof said plurality of photodetectors being spaced along said seconddimension on opposite sides of said one photodetector, and saidcontrolling means selectively providing said separate outputsindividually and in combination, to provide an output signal from saidphotodetector means.